1. Field of the invention
The present invention generally relates to a device including a thermistor element having a characteristic of a positive temperature coefficient of electrical resistivity (referred, hereinafter, to as PTC), where the PTC thermistor element is enclosed in a case. In particularly, the present invention relates to the PTC thermistor device for use in starting an single phase, asynchronous electrical motor in a refrigerator compressor and the like.
2. Description of the related art
The thermistor element of the PTC thermistor device for use in starting an electrical motor is connected to an electrical power source in series with an auxiliary start winding for starting the motor. In motor starting, the temperature of the thermistor element itself of the PTC thermistor device is low so that it remains in a low resistance condition. Consequently, in motor starting, the thermistor element permits a high initial current to be supplied into the auxiliary start winding for starting the motor, so that the motor can start. Subsequently, the thermistor element is heated up and stabilized in a high resistance condition due to the self-heating thereof, which is generated by the current supplied during the running of the motor so that the supply of current to the auxiliary start winding is automatically stopped.
Conventionally, this type of PTC thermistor device is removably coupled with the projected hermetic terminals which are provided for the motor. There is shown one example of such conventional device which is disclosed in U.S. Pat. No. 4,241,370. FIGS. 4 and 5 show, respectively, the above-described example. As it is apparent from FIGS. 4 and 5, this type of device includes a case 1.sub.0 having a pair of openings 2.sub.0, 3.sub.0 in which the "plug-in" type terminals 4.sub.0, 5.sub.0 are, respectively, provided. The corresponding hermetic terminals A and B are, respectively, inserted in the "plug-in" terminals 4.sub.o, 5.sub.o. The "plug-in" type terminals 4.sub.0, 5.sub.0 are, respectively, connected to the corresponding contact terminals 7.sub.0, 8.sub.0 which are accommodated in the case 1.sub.0 to be in contact with the respective electrodes of a PTC thermistor element 6.sub.0. It is to be noted that a reference symbol C in FIG. 4 indicates a common terminal to be connected with a power source. This common terminal C connects a terminal of a main winding (not shown) with the terminal A or B of the auxiliary start winding for starting the motor. As described above, the conventional PTC thermistor device has the openings which are through an outer wall of the case 1.sub.0, therein two hermetic terminals A and B are, respectively, connected with the corresponding "plug-in" terminals 4.sub.0, 5.sub.0. Therefore, the humid air and/or oxidizing gas are able to come easily into the inside of the case 1.sub.0 through the openings. This causes the thermistor element 6.sub.0 to be degraded. Furthermore, the respective contact terminals 7.sub.0, 8.sub.0 are oxidized by the humid air which intrudes into the inside of the case 1.sub.0, thus resulting in that electrically imperfect contact could be caused between the contact terminals 7.sub.0, 8.sub.0 and the electrodes of the thermistor element 6.sub.0. In the conventional PTC thermistor device, there are such disadvantages as described above.
In addition to the above, in the case where the hermetic terminals A and B are inserted from the improper direction to the respective "plug-in" terminals 4.sub.0, 5.sub.0, there happens that the above-mentioned two members are not properly connected with each other. In this case, sparking at a connecting portion of two members may occurs. However, it is difficult to find out whether or not a connected condition of both terminals 4.sub.0, A; 5.sub.0, B is proper since the connecting portion of the two is covered by the outer wall of the case 1.sub.0. So, even if it is improper, an apparatus attaching the PTC thermistor device can be delivered from a factory. Accordingly, the possibility of the above-mentioned situation, i.e., the imperfect contact, can be thinkable as well in the conventional PTC thermistor device because of its structure.